A less obvious use is to track recovery of a large cluster over
time. With enough shards, starting a cluster, or even recovering after
losing a node, can take time (depending on your network & disk). A way
to track its progress is by using this command in a delayed loop:

In this scenario, we can tell that recovery took roughly four minutes.
If this were going on for hours, we would be able to watch the
UNASSIGNED shards drop precipitously. If that number remained
static, we would have an idea that there is a problem.